1. Field of Invention
The present invention relates to a crystal growth furnace comprising a liquid-cooled heat exchanger that is vertically movable beneath a crucible.
2. Description of the Related Art
Crystal growth furnaces are used to produce crystalline materials from melted feedstock material. Feedstock is first melted in a crucible and then re-solidified into crystalline materials, for example into crystalline ingots. Several methods are used to produce crystalline ingots from melted feedstock. For example, the Czochralski (CZ), directional solidification (DSS) and heat exchanger method (HEM) furnaces can all be used to melt silicon feedstock material to produce a crystalline silicon ingot. However, the means by which the melted feedstock is re-solidified into an ingot differ between them. In the CZ method, silicon ingots are produced from melted feedstock by first dipping a silicon seed, suspended from above, into the melt and then slowly “pulling” the partially melted seed up and out of the melt so the silicon begins to cool and solidify from the “top-down” to form a silicon ingot, also known as a boule. The ingot formed is single crystal in nature and highly suitable for semiconductor and photovoltaic applications. However, the relatively small size of the silicon ingot produced by the CZ method, relative to the cost of production, makes this method generally impractical for producing the large quantities and large ingot sizes needed to make silicon wafers for the photovoltaic industry.
Unlike CZ, the DSS and the HEM methods re-solidify molten feedstock material into crystalline ingots from the bottom of the crucible “up”, generally in the same crucible the feedstock was melted in. These methods may be used to produce larger ingot sizes than CZ. Several kinds of crystalline ingots can be produced by the DSS and HEM methods, for example, silicon and sapphire ingots. However, the manner in which molten feedstock material is re-solidified in the DSS and HEM methods differ due to their furnace configurations. Typically in a DSS system, a square crucible containing feedstock is heated, the feedstock is fully melted and then heat from the molten feedstock is allowed to radiate from the entire bottom of the crucible to the water-cooled furnace chamber wall below. This method of cooling the crucible from below produces a temperature gradient in the crucible that promotes the “bottom-up” solidification to form an ingot.
Unlike the DSS method, the HEM method produces a temperature gradient in molten material by positioning a heat exchanger in thermal communication with the crucible bottom to extract heat away from the molten material in a more focused manner. Coolant, for example, certain gases, circulating through the top of the heat exchanger, carries away extracted heat from the crucible bottom to initiate crystallization and promote the “bottom-up” solidification to produce an ingot. Helium is commonly used in furnace heat exchangers as a coolant. However, helium, as with other cooling gases, has no appreciable mass and so has little capacity to absorb large quantities of heat from crucibles containing molten material. As a result, large quantities of gas must be circulated through the heat exchanger to maintain the heat path and flow of heat from the melt, through the crucible and into the heat exchanger to promote and maintain crystal growth.
Unlike gas coolants, liquid coolants have appreciable mass. Water, for instance, is an exceptional liquid coolant because it has appreciable, mass and therefore has significant capacity to absorb large amounts of heat from the crucible. However, its use as a liquid coolant in heat exchangers in crystal growth furnaces has been guarded because any water leak from the liquid-cooled heat exchanger into the furnace chamber could potentially generate significant amounts of steam pressure.
As such, there is a need in the industry to take advantage of the superior cooling capacity of a liquid coolant comprising water over coolant gases used in heat exchangers to grow crystalline materials.